In the basins and valleys adjoining the coast, climate is subject to wide variations within short distances as a result of the influence of topography on the circulation of marine air. The Los Angeles Basin and the San Francisco Bay area offer many varieties of climate within a few miles.  In the Bay area, for example, the average maximum temperature in July is about 64° F. at Half Moon Bay on the coast, 87° F. at Walnut Creeek only 25 miles inland, and 95° F. at Tracy, just 50 miles inland.  Santa Monica Pier, in the Los Angeles area enjoys a normal July maximum of around 75° F., while the average increases to 95° F. at Canoga Park in the San Fernando Valley just 15 miles to the north.

EFFECT OF GENERAL CIRCULATION ON CLIMATE – A dominating factor in the weather of California is the semi-permanent high pressure area of the north Pacific Ocean.  This pressure center moves northward in summer, holding storm tracks well to the north, and as a result California receives little or no precipitation from this source during that period.  In winter, the Pacific high retreats southward permitting storm centers to swing into and across California.  These storms bring widespread, moderate precipitation to the State.  Some of them travel far enough to the south to spread moisture beyond the Mexican border.  When changes in the circulation pattern permit storm centers to approach the California coast from a southwesterly direction, copious amounts of moisture are carried by the northeastward streaming air.  This results in heavy rains and often produces widespread flooding during the winter months.